Well logging method

ABSTRACT

A well logging method measures changes introduced into the circulatory system at or near the drill bit, which changes may be in mass, physical form, materials, chemicals, or energy, providing a determination of permeability, some lithology, possible productivity, and various other useful data needed in drilling, and particularly obtaining such information during actual drilling operation. Difference in quantity of filtrate in the circulating mud and differences in the condition of the mud from interval to interval of bit penetration provide a direct determination of permeability, among other data, and eliminates many errors introduced into other logging systems by extraneous influences, and reflects conditions at the drill bit. One logging system is prepared by recording the changes in the fluid circulatory system which occur at the bit in relation to the incoming fluid circulatory system, and a second logging system is provided by recording data changes from interval to the next succeeding interval. These two types of logs may then be correlated with conventional logs as a valid and positive means of identifying otherwise unknown or unavailable information.

United States Patent [191 Eckels Apr. 9, 1974 Primary Examiner-Jerry W. Myracle Attorney, Agent, or F irmRichard D. Law

[57] ABSTRACT A well logging method measures changes introduced ELT FFE a -5 TENSLEEP AMSDEN MADISON "0" into the circulatory system at or near the drill bit, which changes may be in mass, physical form, materials, chemicals, or energy, providing a determination of permeability, some lithology, possible productivity, and various other useful data needed in drilling, and particularly obtaining such information during actual drilling operation. Difference in quantity of filtrate in the circulating mud and differences in the condition of the mud from interval to interval of bit penetration provide a direct determination of permeability, among other data, and eliminates many errors introduced into other logging systems by extraneous influences, and reflects conditions at the drill bit. One logging system is prepared by recording the changes in the fluid circulatory system which occur at the bit in relation to the incoming fluid circulatory system, and a second logging system is provided by recording data changes from interval to the next succeeding interval. These two types of logs may then be correlated with conventional logs as a valid and positive means of identifying otherwise unknown or unavailable information.

12 Claims, 4 Drawing Figures DELTA LOG WELL-#l DELTA LOG DELTAZ LOG TENS AMSDEN 3,78 MADI 0N"A" 4,060 MADI ON"B 4,294 MADISON "D" 4,360 MADISON"F" INVENTOR ROBERT E. ECKELS ATTORN PATENTEDAPR' 91974 3302.259

SHEET 2 OF 4 wELL 'l DELTAZ LOG DELT'A2 LOG DELTA LOG DELTA L06 3,490 10-63 -20 2 6 I0 50 -lO-2O -6 -202 6 -|0-2O INVENTOR FIG. 2 ROBERT E. ECKELS ATTORNE PATENTEUAPR 9 1914 SHEET u or 4 oOJ [55mm 03 ide w QI INVENTOR ROBERT E. ECKELS BY a 0 ATT RN WELL LOGGING METHOD SPECIFICATION In the completion or abandonment of test wells, drilled for producible fluids, much data must be compiled, interpreted, and then acted upon by the well planners using their best judgment. A great deal of useful data is not presently available until the actual drilling has been suspended, the drilling tools pulled from the hole, and logging devices lowered into the hole. Ideally, information affecting the drilling planners decisions would be in the hands of the planners at the time of bit penetration. Among the types of information which would be most helpful are the nature of the penetrated formations and probable productivity.

Conventional testing and logging of drilling muds and cuttings gives a lithology, an estimate of oil in the cuttings, an estimate of hydrocarbons in the mud, an estimate of gas in the mud, and a general description of the cuttings. At best, using steam distillation and the latest equipment for hydrocarbon logging, it is only possible to estimate the quantity of hydrocarbons in the paraffin range of C through C or C Conventional mud analysis logging generally depends on non-introduction of fluids into the mud stream after an interval of hole has been penetrated, meaning the formation pressure is less than the hydrostatic pressure of the mud. A reverse of this pressure system, obviously, opens the opportunity for potential blow-out. The logging systems now used depend upon gas chromotography, drilling rate, visual inspection of the hole, examination for oil in the mud and cuttings, and various supporting devices determining such data from the cuttings and the mud itself. A shale density log is sometimes also available from this method.

Visual observation of the drilling process at the bottom of the well cannot be performed during the drilling operation. Visual observation of the well is sometimes possible, however, when the drill string is removed from the well, and a television camera or borehole televiewer is lowered into the hole. Also, various types of instruments may be lowered into the well to obtain other data as by radiation. In other logging and data collection systems, a core may be taken so that theac tual formation may be analyzed. Such cores are nearly always cut in conditions of positive hydraulic head in the circulatory system with some flushing possible. In still other instances, a drill stem test may sometimes be made, which is usually conducted by blocking off a portion of the well by seals and the like and analyzing inflowing fluids, where the positive hydrostatic head is established as that of the formation tested.

In wildcat wells, very little of the formations to be encountered is known, even though a general contour has been estimated and types of formation layers are estimated to be present in the area at estimated depths. In such wells, drill stem tests, cores, wireline logs which includes obtaining electrical resistivity, gamma ray tests, neutron tests, sonic tests, and the like, and mud and cuttings analysis are used to try to reduce earlierv estimates to facts. In areas of better known geology, a minimum wireline logging and testing program is usually indicated, sufficient for correlations and completion.

One commonly used well logging technique to determine the presence of hydrocarbons involves treating a portion of the mud to remove included gas or oil and a hot wire gas detector is used to determine the relative amount of gas. More recently a gas chromotograph has been used for hydrocarbon logging. Conventional logging, inspection and testing of drilling mud and cuttings gives a lithology, an estimate of oil in the cuttings,-an estimate of the hydrocarbons in the mud, an estimate of the gas in the mud, and a description of the cuttings providing a general description of the formation through which the drill passes. Porosity of the formation is often determined by laboratory core tests or-by wireline porosity logs which, of course, requires the removal of the drill string prior to conducting such tests.

According to the present invention, there are provided log systems which are called a differential log and a double differential log. These two logs refer to a family of logs which generally refer to differences in filtrate and circulatory fluid conditions. The logs may include differences or changes in differences of filtrate indicator concentration, resistivity, density, pH, volume, loss from a unit of the mud, etc., all of which are of interest to a well geologist, a well programmer, and the comple' tion and production engineer because they relate to conditions at the bit as it penetrates various formations. Also, of interest to the well programmer and engineers, but'of more importance to the mud engineer in identifying immediate mud changes for more timely alteration as needed, are the differential logs relating to mud weight, total mud volume, drilling weight, drilling rate ion concentrations, filter cake data, shear and gel strength, mud viscosity, mud rheology, concentration of lignosulfonates, various indicators, etc. The most important differential log is the permeability log, available from the filtrate volume difference by itself or preferably with other data. Permeability may be quickly identified and is obtained on an essentially quantitative basis.

In the consideration at the rate of filtration from the mud of filtrateinto adjoining formations, it is important to note that the rate of filtration increases towards the bit from the unpenetrated formation, and it is highest at the instant of bit penetration, rapidly falling off to become somewhat asymptotic over a longer time interval after the bit penetration. This filtration consideration, of course, is subject to the formation being permeable, regardless of porosity. Since, except for temperature, the condition of an increment of mud is probably the same when it reaches the bit as when it leaves the surface, in normal circulation, and since the instantaneous filtration rate at the bit is at its highest with positive hydrostatic pressure, and since the return conditions from the bit to the surface are substantially in equilibrium, it follows that the filtrate loss reflected from the mud entering the system and the mud leaving the system (the out mud) are a unit and on that basis has to represent the character of the permeability which received the loss filtrate at the bit depth. In fact, the loss to the fresh-cut sidewall of the hole will be the major filtrate loss due to permeability because that filtrate which is lost ahead of the bit partially re-enters the system as the bit progresses toward any lesser permeability. This may represent a small quantitative error which can be corrected if necessary. A similar error may be introduced from the cuttings volume itself. An alternative to this, is the situation where the bit is drilling a rock which has high porosity, high connate water and low permeability. As this latter rock is drilled, the

connate water is released along with cuttings. into the mud stream. Such released connate water will affect the volume of the filtrate recovery in the out mud as well as its density, resistivity and other properties.

Other variables which become usable by means of the differential logs are indicated as follows:

A. A double differential indicator log, wherein an indicator is introduced into the circulatory system, which is selectively soluble in the liquid of the mud and extraneous to the formations being drilled, and which may be in the form of ions, dyes, fluorescing materials, nuclear materials, magnetic or even such often included mud materials as chrome-lignosulfonate. Changes related to formation hygroscopic absorption may be identified, barring selective reaction rates of ions or variable absorption by surface effects. Otherwise, absorption or even reactions are not serious evaluation problems in that their equilibrium conditions may be sufficient to negate such effects when using the changes of differences technique.

B. Double differential resistivity log and double differential density log. The differences in resistivity and density of the filtrates are important in helping to identify the possible formations penetrated. As the bit penetrates a formation, a certain amount of thematerial is reduced to very fine particle size which may be conducive to solution under the high temperature and pressure conditions. In some instances, the formation salts often remain in solution, where they have not exceeded their saturation conditions, upon reaching the surface. Both conductive and high resistivity salts may be dissolved, salts may be precipitated from the solution of the mud at or near the earths surface, and resistivity and density differences become meaningful.

C. The differential pH or double differential pH log can become very important in control of the mud system for more timely adjustments by the mud engineer. Sharp changes in pH may be the result of the penetration into acid water, etc.

D. Double differential sample temperatures may help v to identify transition zones, where accurate data on temperature changes, used with other curves and particularly filtrate volume differences, which, also, may help to identify possible gas-bearing sections.

E. Double difierential hydrocarbon content. Hydrocarbon content data from existing measurement methods, but incorporating the data from the entering mud, will be substantially more important than the direct out mud readings now used. Such a log should be considerably better where relatively smaller amounts of hydrocarbons are in the mud system.

F. Double differential chrome lignosulfonate log. Lignosulfonate per se in the mud has thus far been extremely difficult to identify quantitatively. Some limited success has come from the determination of the chrome ion; however, the chrome appears to be highly absorbed or reactive and thus introduces errors which have not been resolved. Lignosulfonate, as well as other lignates, concentrations may be determined in spite of turbidity and scatter from gels, lines, and suspended oil particles.

' In making permeability determinations using the technique of filtrate difference, it is important to consider the case that if a rock has no porosity it will also not have permeability. The reverse does not hold true, in that if'a rock has porosity it may or it may not have permeability. While there is a relationship between increased permeability'due to an increase in porosity, the more important parameter for effective production purposes is permeability. Permeability is that communication which is measured by the permeability differential log and it will include permeability from unusual porosity and vertical and horizontal features in communication with other permeability. The importance of porosity is not lessened by these considerations; its value as a source of data defining reservoir capacity may be determined from other sources. Knowledge of permeability, or broadly the ability of a formation to produce, is needed almost at the same time that the bit penetrates the particular formation. This information is, also, available from the double differential permeability log.

The double differential logs are applicable to all exploration and development wells. They are especially valuable in assisting wireline log interpretation. The correlation of permeability to porosity, drilling rate and lithology may be obtained. Where anindicator is used the system provides a means to identify the source of waters in drill stem tests as filtrate or formation water. In using the system it is desirable to have the logging equipment automated and computerized as to read directly with usable units for each parameter involved and in turn collectively fed into data reference banks for quick interpretation.

Included among the objects and advantages of the present invention is to provide a well logging method for determining permeability of the formations through which the drill bit is passing. Another object of the invention is to provide a logging method which measures changes introduced into the circulatory system at or near the drill bit providing a means of determining lithology, possible productivity and various other data needed in drilling. An additional object of the invention is to provide a method of determining permeability while drilling. A still further object of the invention is to provide a method for measuring changes of material, chemicals, energy balances, etc. in the circulatory system of a well drilling operation at or near the drilling bit. A still further object of the invention is to provide a well logging method which eliminates many errors introduced into other logging systems by extraneous influences and accurately reflects conditions at the drill bit.

Other and additional objects and advantages of the invention include a well logging method which correlates conventionally obtained logs with the logs obtained by measuring changes introduced into the circulatory system. A particular object of the invention is to provide a differential log which provides a measurement of change in a particular type of information measured in the circulatory system from a norm, for example a filtrate volume difference, and to provide a double differential log measuring change in an identification system in the circulating system of the well, such as filtrate volume difference change, on an interval by interval basis of the depth of the well being drilled.

These and other objects and advantages of the invention may be readily ascertained by referring to the following description and appended illustrations in which:

FIG. 1 is a composite well logv of a well, identified as Well No. 1, showing a differential log plot of filtrate volume difference Log A;" a double differential log plot of change in filtrate volume difference Log B, and a porosity log Log C;

FIG. 2 is a composite well log of Well No. l,covering the in'tervaia 'rmiraaaeirsmngsisg'pist 6f slan in filtrate density difference Log D; change in filtrate resistivity difference Log E; filtrate density difference Log F and filtrate resistivity difference Log G,

FIG. 3 is a composite well log of a well identified as Well No. 2, showing a differential log of filtrate volume difference, Log H; a log of the change in filtrate volume difference Log 1; and core permeability Log 1; and

FIG. 4 is a composite well log of Well No. 2 for the same in teryal, showing a porosity log Log K; a double differential log of change in filtrate density Log L; a change in filtrate resistivity difference Log M; a filtrate density difference Log N and a filtrate resistivity difference Log 0.

In sampling the circulatory system of a well, according to the invention, various measurements may be made on a batch or a continuous basis, depending upon the type of equipment used for such testing. It is to be understood that in discussing the equipment and techniques herein, which are mostly related to batch systems, the same well logging method is highly adaptable to continuous methods of measuring the various parameters which are measured from the well circulatory system, and in many respects a continuous measurement of the parameters sampling of the various parameters will provide highly useful information which is not presently available in any logging system.

In one form, for example, the filtrate difference determination on a batch basis is nothing more than a continuous monitoring of batch samples on a close time and/or depth increment basis. The batch determination involves taking an in mud sample of any convenient size, for example 200 to 300 ml, recording the time and temperature of the sample, then after sufficient time elapses for the mud increment (from which the in sample is taken) to circulate down the hole, through the bit, and back up the hole to the surface, an out sample is taken along with its temperature and time of sampling. As an optimum, the out" sample is com-' pared to the same increment or portion of the mud circulation system which was sampled for the in sample. This, of course, is determined by the time of circulation which can be determined by using conventional lag-time techniques. In determining the filtrate volume of both the in and out samples, a conventional mud pressure filter press at 100-150 psi and 30 minutes filtering time is used to produce the samples of the filtrate. The difference between the volume obtained from an out sample subtracted from the corresponding in" sample represents one point of filtrate difference. This point is identified as to the depth in the well by taking the time of the in sample collection plus the time needed for the mud to get to the bit (from pumping and the drill pipe data) then applying such clock time from a drillers log for depth. A plot of such points is a difi'erential plot of filtrate volume difference, identified as a delta parameter log. With a series of such pairs of corresponding samples producing a series of points of filtrate difference, the filtrate differences progressively subtracted one from the other obtains a double differential filtrate reading which reflects the change from one reading to the next, and is identified as a delta squared log.

In considering continuous sampling, continuous samples may be taken from a sampling tube from the in" line to the well and from a sampling tube from the out line to the well and these muds pass through continuous filter presses, continuous centrifuges, continuous ultrafuges, or continuous heat or vacuum evaporation with subsequent collection and measurements. In the evaporation prospect, however, any dissolved salts could affect the density and resistivity readings unless some provisions were made for considering the dissolved salts. The continuous method would include a controlled, continuous filtration unit paired with a similar unit with the read-outs of the in sampler being staggered to the circulatory time with automatic differences being recorded to the out sampler, so that the effect is taken at the drill bit. For example, the filtrate differences may be continuously plotted and the filtrate volume difference and the filtrate volume difference change may be continuously plotted on a continuous log of the well.

For purposes of the present application, two test wells were chosen for the differential and double differential logging methods. The two wells, having the data given below, were both field wells in the Oregon Basin Field, Wyo. Both wells used drilling mud, fresh water, oil lignosulfonate, and other chemicals in the circulatory fluids with the mud and chemicals and surveillance provided by two different mud service companies. Both wells were completed as producing development wells and they were drilled on substantially known geology. Samples for the testing, and to correlate with data otherwise obtained, were taken as mud in" samples entering the system at the suction line of the mud pump, and as counterpart samples of mud out from the same mud leaving the system after circulating through the bit. Each out sample was gathered at the box just ahead of the shale shaker. The time of sampling was recorded along with the sample temperature for each sample taken. The time interval for taking of the out sample after the related in sample was obtained was determined from a conventional lag-time interval. From this data and drill pipe flow data, a depth was determined and assigned to each sample or pairs of samples.

In the test given below, the data collected for the well identified as Well No. l was for a drilled interval, and the data collected for the well identified as Well No. 2

was for a cored interval.

Also, in the test data, subtracting the out filtrate from the in filtrate produces either a positive or a negative volume. A positive volume, or increase in volume, means probably that fluid from the mud cake formation has remained in the circulating system and that there is little or no permeability, or since lost circulation material and cuttings were, left in the evaluated mud, some of the lost circulation material was dropped without a corresponding drop in the water phase, or an inverse pressure relation existed at the bottom of the hole in the presence of an aquifer, or fluid was added to the system from high connate water in drilling a relatively low permeability rock, or a combination of these things. A negative volume or decrease simply represents permeability and the amount of decrease has a relation to the actual permeability of the strata being pierced by the drill bit. This, also, means the existence of porosity and a good indication of the amount of permeability.

For the laboratory tests, 200 ml samples of the various samples of mud were subjected to 150 pounds pressure in a pressure filter for 30 minutes each. The volumes of the filtrate were recorded and programmed along with the density of the mud, lab temperature of the sample, resistivity data, sample identification, and well identifications. The filtrate density was also noted. From the recorded data, filtrate differences (delta parameter log) and changes in differences (delta squared log) were calculated, and similar information derived for mud density, filtrate density and resistivity, after the resistivity readings were corrected for temperature and scale. The resulting information from the lab tests and calculations were then plotted on logging strip paper at inches to 100 feet, and these were correlated with the resulting curves of sample geology, drillers log, wireline logs, cores and the like.

' Well No. 1

The resulting curve of the filtrate volume difference, Log A, FIG. 1, for this well drifts in the Tensleep and the top of the Amsden toward a positive volume difference in overall character. This is at least partially due to normal sidewall cake buildup where substantial water is being released back into the circulatory system. A tentative cake effect, zero line is located on the curves from points cross-referenced to I zero changes in filtrate resistivity and density, assuming this might represent an equilibrium. With the cake zero line, more meaning is added to the curve. A second curve, Log B, FIG. 1, is formed by plotting the net change from point to point, which provides a double difference (delta squared log) and this second curve correlates both in character and in quantity with the available logs. Also, considering that if everything else is equal, including other rock characteristics, greater porosity should drill faster; this is suggested by a plot of variations in drilling rate from the bit wear average.

Permeability summary for Well No. l

The character of the permeability curve of the double differential (filtrate permeability), even with relatively few points, is identifiable withthe porosity curves available from the Formation Density Compensated gamma-ray curve, and the Sidewall Neutron Porosity log. Even with some error in sampling methods, lag times or otherwise, the intervals are comparative on a depth basis with other available logs. Similarly, salts and formations are partially identifiable from variables in density and resistivity of the filtrate, intrinsic ions and other data from the filtrate.

In the following tables I and ll, permeability and lithology respectively are determined from the log method of the invention, utilizing a few points in the well:

T BLE .ILYYELLNO- Permeability Porosity PML SNP FDC indicated %Poros FVDC Depth kPorosity Permeability ity IF LS PermealF SS bility 3529 0,5 No 0.0 No 3547 6 No 5 No 3576 -5 No 2 No 32 0.3 No 0.0

3936 1.5 No L5 4086 8 Yes 15 l.7 ml 4088 5 Yes ll l .0 ml 4096 12 No 19 No 4396 2 No 1.3 No 4370 [4,5 Yes 20 5 ml 4374 6 t0 :0] ml 4405 4 No 12 No .i ALl l 7 Yes A ll Q- ml Index FDC Compensated Formation Density Log PML Proximity micro-log, indicated SNP Sidewall Neutron Porosity log FVDC Filtrate volume difference change, Delta Squared log The FDC is determined assuming the formation to be sandstone, and the SNP is determined assuming the formation to be limestone, both are common techniques for logging.

TABLE IL-LITHOLO GY Indicated lithology, percent Mud log, perfrom FDC FDDC, Depth cent lithology vs. SNP grnJcc. FRDC Comments 3,529-". D, 108b, 2088 L 0.022 0. 0008 Salt soln. 3,547.... 70]), 208b, 1088 D, 25L --0. 02 0. 0002 Entry of Ir.

wa er. 3,576.--. 70D, 30Sh 60D, 40L 0.0276 0. 002 Salt soln. 3,780.... 60L, 40811 L 0 0 No change. 3,936.- BOL, 20811 70L, BOSS 0 0 Do. 4,086---. L 66D, 33L 0.02 -0. 0005 Salt soln. 4,088-.-. L 50L, 50D -0. 03 0 Connate water. 4,096 L 50L, 5058 0.045 0.0001 Soln. of nonelect. salts 4,364. L L, 20D 0. 1228 0. 0002 Soln. of

elect. salts. 4,370-..- L 70L, 30D 0.007 0 Soln. of nonelect. salts. 4,374-... L D 0.0043 0. 0002 PPT of nonelect. salts. 4,405-. L 75D, 25L 0. 0117 --0. 0005 -Do. 4,4ll L D +0. 0089 0. 0001 Salt soln.

L=Limestone.

In Table I the PML indicated permeability (PML) correlates well with the permeability determined from the log of the filtrate volume difference change. The double differential filtrate curve of permeability is readily identifiable with porosity (as when there is porosity there may or may not be permeability). Salt bearing formations are identified from the comparison of the change in filtrate density difference (delta squared log) compared to the change in filtrate resistivity difference to provide immediate indications in lithology, as shown in Table ll. Also, the indicated lithology obtained from a comparison between the Formation Density Compensated log and the Sidewall Neutron Porosity log correlates very well with the mud log, considering that dolomite and limestone are similar geologically, frequently having similar densities. Further, the

comparison of the FDDC against FRDC will show fresh water intrusion, precipitation of salts (either electrolytic or non-electrolytic), and other concentration changes.

The determination of permeability indicates a potentially productive zone, which should then be compared to a hydrocarbon log. By using a delta squared log of the hydrocarbon, productive zones are determined. Well No. 2

As pointed out above, some intervals of this well were cored during drilling and during the coring of the well thefcake effect of the mud igWell No. 1 was ngt noticed. Data in table III were picked where they were significant and these were comparable with the definitive character of other logs taken.

In the following table, permeability as indicated from method of the invention is compared to permeability found by core analysis in a laboratory for some depths of the well:

continuous methods. Computer applications to such methods provide several advantages such as on-the- TABLE I \,s (Wcll#2) ll, LS,

FIX lML Core l VlHJ percent indicated Core permcpHI'muhVl) per- Depth porosity permeability porosity ability FDDC ability mnuhi ity 8. 10.1 0.9 0.0441 N N0. 23. 5 22. 3 83 -0. 0254 -9. 8 Yes. 7 25. 6 11.0 0. 1213 2. 4 Yes. 3 7 01 0. 0071 No Yes. 7 10. 9 0. 5 +0. 0231 5. 4 Yes. 12. 5 7. 2 0.1 0. 0209 0. 1 Slight. 12 12. 5 0. 1 +0. 0631 2. 5 Yes.

4 3. 7 0. 1 +0. 0319 No Slight. 4 4. 8 0. 8 -0. 0412 N0 N0. 4 1.4 0.1 0. 0713 No No. 15 14.1 32 +0065 2.3 Yes. 4 11. 1 7O 0. 0919 3. 8 Yes. 4 4 0.2 +0. 0619 N 0 Yes.

INDEx.The headings have the same meaning as in Table II above.

' hftable lll, increases in filtrate volume are indicated by positive values or No. permeability and decreases are indentified as negative values, or permeability. To overcome some difficulty in interpretation, an algebraic difference of the differences may be considered from plotted points for filtrate, density, and resistivity against depth. The permeability determined in Well No. 2 is compared with available wirelin e logs; these show a good correlation.

In both of the wells, the laboratory data and the calculations were plotted showing reasonable correlation with known logs. in plotting the data, the differences have been plotted allowing interpretation on a point-topoint (depth) basis with reference against each preceding point and a nominal mud line. To reduce some of the errors and thus provide a valid interpretation as the mud cake line varies with downhole conditions, an algebraic difference of the differencesmay be plotted for the points for filtrate volume, density, resistivity, and other parameters against depth.

in considering the resulting data from the tests as points on a depth graph, there is defined a curve of permeability change. The curve may be considered for permeability, however, some better meaning is given the curve by establishing a zero line which may be takenas a mudcakeline based upon the mud cake coir stantly being formed. Such a mud cake effects a positive difference in the measured filtrates, causing the curve to drive to the positive side. The mud cake zero line will change from time to time but may be generally located by using the point of filtrate volume difference which corresponds in depth with no difference in density or resistivity of the filtrate. A very important aspect of the invention is the recording and the use of the change in differences or the difference in differences. By subtracting the difference in a filtrate parameter from its immediate predecessor the resulting difference effectively starts each point from the zero reference, noting actual change from point to point. Many errors which might otherwise be contributed from external influence into the circulatory system as well as many sources of error effecting single points become either unimportant or completely eliminated. The curve resulting from the points of the double differential is easy to interpret and to correlate with other data.

The desired samples may be taken directly from pressure filters, centrifugal filters, ultracentrifuges, evaporation condensates, or the like, or they may be determined indirectly as with bulk density correlations of the I samples themselves. The data is more meaningful using spot analysis of the drilling Utilizing the inventiomformations are identifiable as relatively water-dry, or hygroscopic, and with permeability and porosity, they may be expected to be potentially hydrocarbon productive if hydrocarbons are found in the formations. Such data are readily available from the method of the invention. Furthermore, the method of the invention, that is, the use of the double differential logs, generally applies to mining drilling investigations which utilize suitable circulatory systems, water wells seeking permeability and other geological wells as well as those of the petroleum industry.

I claim:

1. A method for logging information obtained from tests during well drilling operation wherein a drilling fluid flows from a well head to a drill bit during drilling operation, comprising conducting a series of pairs of tests to obtain a set of data for each of a series of depth location points in the well; the first of each of said pair of tests being conducted on an increment of entering drilling fluid providing a standard datum for each particular point, and the second of each of said pair of tests being conducted on said increment of entering drilling fluid after passing the drill bit providing a reference datum for each particular point; subtracting the reference datum for each particular point from said standard datum of that point to provide a series of differences for said series of depth location points; subtracting each difference from the preceding difference to provide a series of changes of differences for each particular depth location point below an initial starting depth location point.

2. A system for logging information according to claim 1 wherein said tests are conducted during the drilling to obtain such logging during actual drilling operations with the drill string in the well.

3. A system for logging information according to claim 1 being further characterized by plotting the resultant differences and changes of differences on a well chart. V 1

4. A method for logging information according to claim 1 wherein the drilling of the well includes a coring operation and conventional tests are conducted on the resulting core, obtaining including standard datum and reference datum for each depth location point.

5. A method for logging information obtain from tests during well drilling operation wherein a drilling fluid flows from a well head to a drill bit during drilling operation, comprising conducting a series of pairs of tests to obtain a series of data for a series of depth location points in the well; the first one of said pair of tests being conducted on an increment of entering drilling fluid providing a standard datum for each particular point, and the second one of said tests being conducted on said increment of entering drilling fluid after passing the drill bit providing a reference datum for each particular point; subtracting the reference datum for each particular point from said standard datum of that point to provide a series of differences; and plotting said differences on a well chart at each corresponding depth location point along with other conventional test data for each such point. 7

6. A method for logging information obtained from tests conducted on circulating fluid during a well drilling operation wherein a circulating fluid circulates from a well head to a drill bit during drilling operations comprising conducting a plurality of pairs of tests to obtain for each depth location point in the well a standard datum of an entering increment of circulating fluid and a reference datum of said entering increment after passing the drill bit for each such depth location point; subtracting each reference datum from the corresponding standard datum for each depth location point to thereby provide a series of differences for each pair of series of tests for each depth location point; subtracting each difference from its immediate preceding difference in the series of differences to thereby provide a series of changes of differences for each series of differences at each depth location point below an initial depth location point for said tests.

7. A method for logging information according to claim 6 wherein the volume of liquid in each drilling mud sample is determined to obtain said standard datum and reference datum for each particular depth location point.

8. A system for logging information according to claim 7 wherein the quantity of liquid in said circulating fluid immediately prior to its contact with the drilling bit is the standard datum and the quantity of liquid in said circulating fluid after contact with the drilling bit is the reference datum.

9. A well logging method for determining permeability of drilled formation during drilling of the well using a liquid circulating system, comprising periodically sampling ingoing circulatory liquid to provide a first series of samples having one standard datum sample for each depth location point; sampling outgoing circulation liquid on a depth of well basis to provide a second series of samples providing one reference datum sample for each depth location point; correlating said first series of samples with the second series of samples to provide pairs of in and out samples corresponding to an in sample immediately prior to its reaching a drill bit and an out sample immediately after leaving the drill bit; extracting filtrate from each said pair of samples under essentially identical conditions; comparing the quantity of filtrate of each said pair of samples to provide a difference between each standard datum sample each of a series of depth location points in the well;

sampling outgoing circulation liquid on a depth of well basis to provide a second series of samples having one reference datum sample for each depth location point; correlating said first series of samples with the second series of samples to provide pairs of in and out samples corresponding to an in sample immediately prior to its contact with the drill bit and'an out sample immediately after the circulating liquid leaves the drill bit; extracting filtrate from each said pair of samples under essentially identical conditions; comparing the quantity of filtrate of each said pair of samples to provide a difference between each standard datum sample and its corresponding reference datum sample, plotting the difference between each pair of samples on a well log for each depth of location point in the well; subtracting each subsequent difference from the preceding difference to provide a series of filtrate volume difierence changes for each depth location point below an initial point for said sampling.

11. A well logging system according to claim 10 wherein a number of different tests are conducted on said samples of circulating liquid, the results of each pair of such tests are subtracted from each other, and the resulting difference is subtracted from the difference of the preceeding tests to provide a series of changes of differences.

12. A well logging system according to claim 10 wherein said series of filtrate volume difference changes are plotted on the well'chart along with said differences between each pair of samples for each depth location point. 

1. A method for logging information obtained from tests during well drilling operation wherein a drilling fluid flows from a well head to a drill bit during drilling operation, comprising conducting a series of pairs of tests to obtain a set of data for each of a series of depth location points in the well; the first of each of said pair of tests being conducted on an increment of entering drilling fluid providing a standard datum for each particular point, and the second of each of said pair of tests being conducted on said increment of entering drilling fluid after passing the drill bit providing a reference datum for each particular point; subtracting the reference datum for each particular point from said standard datum of that point to provide a series of differences for said series of depth location points; subtracting each difference from the preceding difference to provide a series of changes of differences for each particular depth location point below an initial starting depth location point.
 2. A system for logging information according to claim 1 wherein said tests are conducted during the drilling to obtain such logging during actual drilling operations with the drill string in the well.
 3. A system for logging information according to claim 1 being further characterized by plotting the resultant differences and changes of differences on a well chart.
 4. A method for logging information according to claim 1 wherein the drilling of the well includes a coring operation and conventional tests are conducted on the resulting core, obtaining including standard datum and reference datum for each depth location point.
 5. A method for logging information obtain from tests during well drilling operation wherein a drilling fluid flows from a well head to a drill bit during drilling operation, comprising conducting a series of pairs of tests to obtain a series of data for a series of depth location points in the well; the first one of said pair of tests being conducted on an increment of entering drilling fluid providing a standard datum for each particular point, and the second one of said tests being conducted on said increment of entering drilling fluid after passing the drill bit providing a reference datum for each particular point; subtracting the reference datum for each particular point from said standard datum of that point to provide a series of differences; and plotting said differences on a well chart at each corresponding depth location point along with other conventional test data for each such point.
 6. A method for logging information obtained from tests conducted on circulating fluid during a well drilling operation wherein a circulating fluid circulates from a well head to a drill bit during drilling operations comprising conducting a plurality of pairs of tests to obtain for each depth location point in the well a standard datum of an entering increment of circulating fluid and a reference datum of said entering increment after passing the drill bit for each such depth location point; subtracting each reference datum from the corresponding standard datum for each depth location point to thereby provide a series of differences for each pair of series of tests for each depth location point; subtracting each difference from its immediate preceding difference in the series of differences to thereby provide a series of changes of differences for each series of differences at each depth location point below an initial depth location point for said tests.
 7. A method for logging information according to claim 6 wherein the volume of liquid in eacH drilling mud sample is determined to obtain said standard datum and reference datum for each particular depth location point.
 8. A system for logging information according to claim 7 wherein the quantity of liquid in said circulating fluid immediately prior to its contact with the drilling bit is the standard datum and the quantity of liquid in said circulating fluid after contact with the drilling bit is the reference datum.
 9. A well logging method for determining permeability of drilled formation during drilling of the well using a liquid circulating system, comprising periodically sampling ingoing circulatory liquid to provide a first series of samples having one standard datum sample for each depth location point; sampling outgoing circulation liquid on a depth of well basis to provide a second series of samples providing one reference datum sample for each depth location point; correlating said first series of samples with the second series of samples to provide pairs of in and out samples corresponding to an in sample immediately prior to its reaching a drill bit and an out sample immediately after leaving the drill bit; extracting filtrate from each said pair of samples under essentially identical conditions; comparing the quantity of filtrate of each said pair of samples to provide a difference between each standard datum sample and its corresponding reference datum sample, and plotting said difference between each pair of samples on a well log for the depth of well for each pair of samples.
 10. A well logging method for determining permeability of drilled formation during drilling of the well using a liquid circulating system, comprising periodically sampling ingoing circulatory liquid to provide a first series of samples for a standard datum sample for each of a series of depth location points in the well; sampling outgoing circulation liquid on a depth of well basis to provide a second series of samples having one reference datum sample for each depth location point; correlating said first series of samples with the second series of samples to provide pairs of in and out samples corresponding to an in sample immediately prior to its contact with the drill bit and an out sample immediately after the circulating liquid leaves the drill bit; extracting filtrate from each said pair of samples under essentially identical conditions; comparing the quantity of filtrate of each said pair of samples to provide a difference between each standard datum sample and its corresponding reference datum sample, plotting the difference between each pair of samples on a well log for each depth of location point in the well; subtracting each subsequent difference from the preceding difference to provide a series of filtrate volume difference changes for each depth location point below an initial point for said sampling.
 11. A well logging system according to claim 10 wherein a number of different tests are conducted on said samples of circulating liquid, the results of each pair of such tests are subtracted from each other, and the resulting difference is subtracted from the difference of the preceeding tests to provide a series of changes of differences.
 12. A well logging system according to claim 10 wherein said series of filtrate volume difference changes are plotted on the well chart along with said differences between each pair of samples for each depth location point. 